Continuous separation processes are commonly used for the selective adsorption of para-xylene from a mixture of C8 aromatics. Generally, the processes use a solid adsorbent that preferably retains the para-xylene in order to separate the para-xylene from the rest of the mixture. Often, the solid adsorbent is in the form of a simulated moving bed, where the bed of solid adsorbent is held stationary, and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds or modules. The shift in the locations of the liquid input and output in the direction of the fluid flow through the bed simulates movement of the solid adsorbent in the opposite direction.
Moving the locations of the liquid input and output is accomplished by a fluid tracking device known generally as a rotary valve, which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations by directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time or hold period, the rotary valve advances one index to the next valve position and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to the next valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle.
The principle liquid inputs and outputs of the adsorbent system include four separate streams, namely the feed, the extract, the raffinate, and the desorbent. Each stream flows into or out of the adsorbent system at a particular flow rate, and each rate is independently controlled. The feed, which is introduced to the adsorbent system, contains the para-xylene that is to be separated from the other components in the feed stream. The desorbent, which is introduced to the adsorbent system, contains a liquid capable of displacing feed components from the adsorbent. The extract, which is withdrawn from the adsorbent system, contains the separated para-xylene, which was selectively adsorbed by the adsorbent, and the desorbent liquid. The raffinate, which is withdrawn from the adsorbent system, contains other C8 aromatic components of the feed that are less selectively adsorbed by the adsorbent, and desorbent liquid.
The four principal streams are spaced strategically throughout the adsorbent system and divide the sub-beds into four zones, each of which performs a different function. Zone I contains the adsorbent sub-beds located between the feed input and the raffinate output, and the selective adsorption of the para-xylene takes place in this zone. Zone II contains the adsorbent sub-beds located between the extract output and the feed input, and the desorption of components other than the para-xylene takes place in this zone. Zone III contains the adsorbent sub-beds located between the desorbent input and the extract output, and the para-xylene is desorbed in this zone. Finally, Zone IV contains the adsorbent sub-beds located between the raffinate output and the desorbent input. The purpose of zone IV is to prevent the contamination of the para-xylene with other components.
When a larger scale separation is required, the volume required for the solid adsorbent and process flow rate are increased to meet the production rate requirement. The combination of the increase of flow rate, larger bed height and compression stress as a result of packing of the solid adsorbent leads to a high pressure drop across the adsorbent bed. Rigid packing of the solid adsorbent or large size adsorbent particles in the bed can reduce the pressure drop, but there is a tradeoff in inferior process performance.
Accordingly, it is desirable to provide improved methods and systems for separating para-xylene. It is further desirable to provide such methods and systems that are scalable depending on production requirements without suffering an increase in pressure drop or a loss in performance. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.